Why do some birds’ eggs have patterns?

Many birds' eggs have patterning on them. Mike Toms explains why.

A robin's nest contained blue patterned eggs. © Marcia Straub/Getty

While birds that nest on the ground or in exposed locations tend to have strongly patterned eggs, those of most cavity-nesting species are plain white.

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Ducks and pigeons, which cover their eggs when away from the nest and whose females sit from the first egg, also have unpatterned eggs. This suggests that the shell markings provide camouflage.

There are exceptions, however. The eggs of cavity-nesting tits, for example, bear subtle red-brown markings, suggesting patterning might serve other purposes.

In some species the markings have been linked to deposits that reinforce structural integrity; in others patterning may be a sexually selected signal through which a female can advertise the quality of both herself and her imminent nestlings.

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